Hollow cathodes are typically in the form of metal tubes through which a primary gas to be ionized can flow, the gas being ionized by electrons emitted within the cathode and accelerated to a downstream anode. One type of hollow cathode utilizes a heater to heat a low work function insert in the cathode to a temperature at which electrons are emitted. Such devices often include a high voltage tickler electrode to start an arc near the cathode end in case the electrons emitted by the low work function insert material are not sufficient to start the arc. Several problems occur with this type of cathode, including a lack of reliable startup, startup times of up to several minutes due to the need to preheat the cathode, the susceptibility to poisioning of the low work function material when it is exposed to air during maintenance, and the possibility of plugging up the cathode by sputtered material generated by the high voltage tickler and insert.
Another common type of hollow cathode, which does not use a low work function insert, employs a radio frequency discharge device for initiating an arc between the cathode and anode to heat the cathode tube to electron emitting temperatures. Although such hollow cathode devices are fairly reliable, they operate at very high input power levels (typically several kilowatts) and cannot be made to operate stably at lower levels such as tens or hundreds of watts. In addition, the radio frequency discharge device adds to the complexity of the hollow cathode device. A hollow cathode device which could start rapidly and reliably, which could operate effectively at moderate power levels, and which was of relatively simple construction, would be of considerable value in a wide range of applications that utilize hollow cathode devices.